Linear actuators, such as are used in rugged service industries such as turf care, specialty vehicles, agriculture, service vehicles, construction vehicles, and material handling, typically use a motor (electric or hydraulic), a gear box of a specified ratio, and a screw and nut combination to extend and retract a load. To provide the strongest column load capability and load transfer, a steel screw assembly is designed to pass through an aluminum or zinc die cast gear box, and be secured at a hardened steel end fitting mounting bracket.
In such a screw type linear actuator, the screw must be capable of turning while secured to the end fitting during operation. This ability is provided through a radial thrust bearing and thrust washers. Typically, the thrust washer is affixed to the screw shaft and rotates between the thrust washers that are coupled to the housing to hold the screw shaft in its axial position while transitioning the load. Most linear actuator designs rely on bronze thrust bearings to transfer the actuator load through the drive screw to the end fitting attachment point. However, this arrangement of the thrust bearing and thrust washers causes the axial load on the thrust shaft to be transferred radially to the gearbox housing structure. This requires that the gear box housing structure be designed to carry this load as well as axial shock loads which will be transferred during transient operation of the linear actuator. As a result, stronger gearbox housings are required at a substantially increased cost.
As will be apparent to those skilled in the art, the application of the linear actuators in the rugged service industries identified above subjects the actuators to harsh, all weather environments. Because of these harsh all weather operating conditions, the extension rod end support must be sealed or otherwise close-fitting to the extension rod of the linear actuator to prevent rain, dirt, etc. from entering the linear actuator assembly and potentially fouling the gears or otherwise damaging the internals of the linear actuator. Further, the extension rod between the end fitting coupled to the load and the ball nut within the linear actuator is sealed to prevent the debris from entering the screw portion within the extension rod.
Unfortunately, in current designs these foreign object exclusion requirements have resulted in several problems. For example, the requirement that the rod end support bearing be closely fitting to the extension rod to prevent the ingestion of foreign debris results in increased heat generation at this point of contact as the tube is extended and retracted therethrough. In extreme operating environments, this increased heat may cause binding between the end rod support bearing and the extension rod such that the efficiency of the extension and retraction of the linear actuator is decreased. Further, the increased friction tends to further increase the heating of the bearing and extension rod, and may result in damage to the exterior surface of the extension rod with continued operation during such conditions.
Another problem relates to the fact that within the sealed extension rod there is a rolled screw end configuration with integral rod support that provides the internal support for the extension rod in coordination with the external end support bearing just discussed. However, this configuration creates two separate cavities within the extension rod that are separated by the rolled screw end configuration with integral rod support bearing. As the linear actuator extends and retracts this tube, the pressure in these two cavities is greatly varied.
Specifically, as the tube is extended, the pressure within one cavity is increased as the gas therein is compressed between the end of the tube and the integral rod support bearing. On the other side of this support bearing, the pressure is greatly reduced as the volume increases between that end of the tube and the internal support bearing. The pressure in each of these two cavities tends to increase the overall load on the linear actuator, which also results in a reduction in efficiency and the generation of heat within the tube. While the pressure differentials will often equalize themselves once the linear actuator reaches its stationary position because the two cavities are not completely sealed from one another by the integral bearing, or between themselves and the internal cavity of the linear actuator between the steel screw and the end nut, the problem will again reappear when the linear actuator is extending or retracting the load until a new stationary position is reached and the two cavities are allowed to equalize their pressure.
A related problem relates to the lubrication within the tube. That is, since the internal rod support bearing is closely fitted to the inner diameter of the extension rod, lubrication within the extension rod cannot move between these two cavities. As a result, and especially during extended operation with the linear actuator and its extended position, the lack of lubrication in the extended end cavity can result in problems in operation, including increased friction, and heat generation.
Another problem existing with current linear actuators relates to the static load holding brake which may be required for particular applications. Specifically, typical static load holding brakes utilize a bearing between two washers, one coupled to the shaft and one to the brake assembly. As pressure from the load pushes on the linear actuator extension rod, this pressure presses the first washer against the wear bearing, which in turn is pressed against the second brake washer. As the friction between these surface connections increases with the increasing pressure, the brake is engaged to stop any further back driving of the extension rod. However, the wearing of these surfaces results in dust building up on the surfaces, which affects the accuracy of the brake and the load at which braking occurs, and can result in slippage once the brake has been engaged.
Therefore, there exists a need in the art for a linear actuator that can withstand the extreme environmental conditions while overcoming the above and other problems existing in the art.